This invention is directed generally to ionography, and more specifically, to electroreceptors for ionographic imaging.
In ionography, latent images are formed by depositing ions in a prescribed pattern onto an electroreceptor surface. The ions may be applied by a linear array of ion emitting devices or ion heads, creating a latent electrostatic image. Alternatively, the electroreceptor surface may be charged to a uniform polarity, and portions discharged with an opposite polarity to form a latent image. Charged toner particles are then passed over these latent images, causing the toner particles to remain where a charge has previously been deposited. This developed image is sequentially transferred to a substrate such as paper, and permanently affixed thereto.
U.S. Pat. No. 4,404,574 to Burwasser et al discloses an electrographic printing system wherein a latent image is projected onto a dielectric record member. The dielectric record member is a clear, transparent, flexible film which comprises a resin film base, a conductive layer on the base, and a dielectric layer thereon. The dielectric layer may be provided with an "anti-blocking" material which enables the film member to be unrolled from a roll holder and transported across an energized electrode. The "anti-blocking" material has no electric function, and is added so that the dielectric coating does not stick to the backside of the substrate when the film is rolled up. The "anti-blocking" material is suspended in the dielectric layer, and may be high density polyethylene or synthetic silica. The member is different from reusable ionographic image receivers in that the latent image is permanently fixed to the member.
Ionography is, in some respects, similar to the more familiar form of imaging used in electrophotography. However, the two types of imaging are fundamentally different. In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The electrophotographic plate is insulating in the dark and conductive in light. The radiation therefore selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. Thus, charge is permitted to flow through the imaging member. The electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
Electrophotographic imaging members may be provided in a number of forms and may be provided with overcoatings for protecting the imaging member. For example, U.S. Pat. No. 4,006,020 to Polastri discloses an overcoated electrostatographic photoreceptor. The disclosed overcoating comprises a first polymer which is an addition polymerization product of methyl methacrylate, n-butylacrylate, and acrylic or methacrylic acid, and a second polymer which is an addition polymerization product of styrene and maleic anhydride. U.S. Pat. No. 4,472,491 to Wiedemann discloses an electrophotographic recording material comprising a transparent protective layer comprised of an acrylated binder. U.S. Pat. No. 4,260,671 to Merrill discloses a photoconductive member which is provided with a polycarbonate overcoat.
Protective overcoats for electrophotographic imaging members also include silicone overcoats. For example, U.S. Pat. No. 4,770,963 to Pai et al discloses a photoresponsive imaging member comprising a first overcoating layer of nonstoichiometric silicon nitride, and a second overcoating layer of a silicone-silica hybrid polymer. U.S. Pat. No. 4,565,760 to Schank discloses protective overcoatings for photoresponsive imaging members comprising a dispersion of colloidal silica and a hydroxylated silsesquixone in an alcoholic medium. U.S. Pat. No. 4,439,509 to Schank discloses electrophotographic imaging members comprising a coating of a cross-linked siloxanolcolloidal silica hybrid material which may be prepared by hydrolyzing trifunctional organosilanes and stabilizing the hydrolyzed silanes with colloidal silica.
U.S. Pat. No. 4,743,492 to Wilson discloses a primer-topcoat system for various substrates. A primer of a mixture of an acrylic resin and an epoxy compound derived from the condensation product of epichlorohydrin and bisphenol A or bisphenol AF is provided with a topcoat of polyvinyl fluoride. The use of the primer-topcoat system is not disclosed as being for electrophotographic or ionographic applications.
Ionographic imaging members differ in many respects from the above-described and other electrophotographic imaging members. The imaging member of ionographic devices is electrically insulating so that charge applied thereto does not disappear prior to development. Charge flow through the imaging member is undesirable since charge may become trapped, resulting in a failure of the device. Ionographic receivers possess negligible, if any, photosensitivity. The absence of photosensitivity provides considerable advantages in ionographic applications. For example, the electroreceptor enclosure does not have to be completely impermeable to light, and radiant fusing can be used without having to shield the receptor from stray radiation. Also, the level of charge decay (the loss of surface potential due to charge redistribution or opposite charge recombination) in these ionographic receivers is characteristically low, thus providing a constant voltage profile on the receiver surface over extended time periods.
However, ionographic imaging members generally suffer from a number of disadvantages. In an ionographic machine, the electroreceptor comes into contact with development and cleaning sub-systems. Also, paper contacts the surface of the electroreceptor in the transfer zone. Thus, an electroreceptor material which has good electrical properties for ionographic applications, i.e. electrically insulating, may be triboelectrically incompatible with the sub-systems of the ionographic machine. For example, a particularly good electroreceptor dielectric material may be incompatible with toner contact because of high triboelectric charging. This incompatibility leads to, among other problems, cleaning failures because of the poor toner release properties of the dielectric material.
A further problem with many ionographic imaging members involves high charge decay and charge trapping. Materials having a high dielectric constant and good toner release properties may suffer from high surface charge decay and charge trapping. For example, materials having a high dielectric constant, such as polyvinyl fluoride, have high charge decay rates and bulk charge trapping.
It is also desirable for exposed surfaces of a dielectric receiver to have good wear, abrasion and scratch resistant properties. Organic film forming resins used in the dielectric imaging layer are subject to wear, abrasions and scratches which adversely affect the response of the dielectric receiver.
The above and other problems limit the use of various materials in ionographic charge receivers. The problems are further complicated in that there are very few materials with high dielectric constants which have the desirable properties for ionographic imaging.